This photograph, taken from the International Space Station (ISS), shows the north coast of Trinidad and a series of subtle, interacting arcs in the southeastern Caribbean Sea.

These are known as “internal waves,” the surface manifestation of slow waves that move tens of meters beneath the sea surface.

An internal wave propagating on the interface between two layers.

The undisturbed sea level is indicated by the yellow line.

Water particles are shown as yellow and magenta dots.

Yellow dots sit in the middle of the water column and move only up and down.

Magenta dots sit at the top and bottom of the water column and move only in the horizontal. - courtesy of Matthias Tomczak-

Watch the animation and concentrate on one of three aspects at a time:

By watching a yellow dot you can see how a water particle in the
middle of the water column moves up and down, but does not move
horizontally, as the wave passes through.

By selecting a
particular magenta dot at the bottom of the water column and watching it
you can see how a water particle moves back and forth horizontally as
the wave passes. By comparing it with the movement of a particular dot
above it you can see that at any one location, particles at the top and
bottom of the water column move always in opposite direction.

By
watching groups of magenta dots you can see that convergences (where
water particles cluster together) and divergences (where particles are
spread out) follow the wave, and that convergences are always located
where the respective layer is thickest, while divergences are found
where the layers are thinnest.

Internal waves produce enough of an effect on the sea surface to be seen from space, but only where they are enhanced due to reflection of sunlight, or sunglint, back towards the International Space Station.

The image shows at least three sets of internal waves interacting.
The most prominent set (image top left) shows a packet of several waves moving from the northwest due to the tidal flow towards the north coast of Trinidad.
Two less prominent, younger sets can be seen further out to sea.
A very broad set enters the view from the north and northeast, and interacts at image top center with the first set.
All the internal waves are probably caused by the shelf break near Tobago (outside the image to top right).
The shelf break is the step between shallow seas (around continents and islands) and the deep ocean. It is the line at which tides usually start to generate internal waves.
Over the island of Trinidad, the heating of the land surface sets off the growth of cumulus clouds.
Off the coast, a light blue northwest-southeast trending plume at image center is sediment embedded in the Equatorial Current (also known as the Guyana Current).
The current is transporting material to the northwest—in almost the opposite direction of the internal waves.
The current flows strongly from east to west around Trinidad, all the way from equatorial Africa, driven by year-round easterly winds.
Seafarers in the vicinity of Trinidad are warned that the current—and its local reverse eddies—make navigation complicated and sometimes dangerous for smaller craft in these waters.

Envisat SAR image shows internal waves in the Strait of Gibraltar

As water flows into and out of the Mediterranean, two currents are formed in the strait.

An upper layer of Atlantic water flows eastward into the sea over a lower layer of saltier and heavier Mediterranean water flowing westward into the ocean. The lower current is called the Mediterranean Outflow water.

As it leaves the Mediterranean near the strait's western end, it flows over a sudden rise in the sea floor, generating a series of internal waves. Internal waves are not directly visible to the observer because they do not result in large undulations on the sea surface; instead, they induce a horizontal surface current, which changes the surface roughness of the sea. From space, internal waves can be detected very efficiently using Synthetic Aperture Radar (SAR) instruments that are sensitive to changes in the small-scale surface roughness on the ocean surface. Internal waves in this image show up as a semi-circular rippled pattern east of the strait's entrance in the Mediterranean Sea.

Additional sets of internal waves generated in the Atlantic Ocean are visible as dark pink lines on the western side of the strait.